They have been spotted from the survey ships of oil exploration companies in the "Atlantic frontier", the region to the north-west of Scotland likely to be the next big oil production area.
Five of the animals, which can be 100ft long and weigh more than 200 tons, were seen last year, according to an unpublished report of the Joint Nature Conservation Committee, the government agency which looks after wildlife for Great Britain.
"It's fantastically good news," said Mark Simmonds, marine biologist with the Whale and Dolphin Conservation Society.
"World-wide they were almost extinct and they had certainly vanished from sight in the North Atlantic in recent decades."
Indications that a population of blue whales was inhabiting the waters west of Scotland first came two years ago from the United States navy's top-secret Sound Surveillance System, which once tracked Soviet submarines for hundreds of miles across the Atlantic using seabed hydrophones down to 10,000 feet.
As well as the sounds made by submarines, the navy also picked up a great deal of whale song, the deep noises the animals make, and American zoologists picked out the voice of the blue whale among them.
The animals were first seen in 1996, when two were spotted, according to the report's author, marine biologist Carolyn Stone, who saw one herself.
"They have a very distinct tall, slender blow [the spout] which can be up nine metres tall, and a small fin for their very large size," she said.
But before the sightings from the seismic survey ships, which fire sound waves at the seabed to search for oil, there had been none for many years, the last recorded being a single sighting off north-west Ireland in May 1977.
The whaling slaughter in the early decades of this centuryreduced the blue whale population in the North Atlantic, once hundreds of thousands, to an estimated 3,000 today.
"It is very significant but it is too early to say if it indicates any sort of population recovery," Mr Simmonds said.
"It also emphasises the importance of this area of the ocean for whales, and the importance of controlling the new industrial activities that are taking place in it."
Blue whales are the largest animals ever to have lived on earth, bigger than any of the dinosaurs.
At birth, a calf is more than 20ft long and weighs two and a half tons; when fully grown, the animal's heart is the height of a tall man, weighs as much as a horse and pumps blood through arteries so big a child could crawl through them.Reuse content